Device for a transparent pipe intended for optical counting and measuring

ABSTRACT

A circular, transparent pipe device for optically counting and measuring objects carried with a fluid within the pipe by means of light beams exiting through the wall of the pipe. The outer radius of the pipe equals or nearly equals the product of the inner radius of the pipe ( 1 ) and the ratio of the light refraction index of the fluid within the pipe to the light refraction index of the medium surrounding the pipe. A camera or other apparatus is located at a camera point at a mean distance from the pipe between distal and proximal points of intersection of light beams hitting the internal wall of the pipe to sense the light beams.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/NO02/00119, filed Mar. 22, 2002, whichinternational application was published on Oct. 10, 2002 asInternational Publication WO 02/079721. The International Applicationclaims priority of Norwegian Patent Application 20011619, filed Mar. 29,2001.

FIELD OF THE INVENTION

The invention relates to a device for a transparent pipe, arranged toallow observation, counting or measuring of objects carried through thepipe by a fluid.

BACKGROUND OF THE INVENTION

The invention was made in connection with the counting and measuring offish carried with water within a pipe, the counting and measuring beingcarried out by means of an optical apparatus which comprises a cameraand is connected to a computer for analysing camera images. The presentdescription is essentially directed towards this field of application,without the intention of being limiting. The counting/measuringapparatus as such is not part of the invention.

It is known to provide windows in a pipe wall to allow counting andmeasuring of objects passing. It is often preferred to mount atransparent pipe section in the pipe because that facilitatesobservation from several sides and because it makes it easier to havethe necessary light where liquid and objects are to be observed.

Light refraction leads to the distortion of objects within the pipe whenthey are observed from the outside, and this is destructive to opticalmeasuring, for example the measuring of the area of the objects orpattern recognition.

In particular, regions next to the centre line of the pipe are subjectto distortion.

To avoid said distortion, it is common to use a transparent pipe sectionwith at least one plane transparent side, through which the objects maybe observed; typically a rectangular pipe section is used.

There are several drawbacks to mounting a rectangular pipe section to anordinary circular pipe. A transition between a round and a rectangularcross-section increases the costs, and the dimensions of the pipesection will be unduly large. Further, turbulence or air pockets are aptto occur, interfering with the measurements.

SUMMARY OF THE INVENTION

The object of the invention is to provide a transparent pipe section ofa circular cross-section for observation, counting and measuring ofitems carried through the pipe section, in which the optical distortionis also reduced.

The object is achieved through features as specified in the followingdescription and subsequent claims.

According to the invention a circular transparent pipe is used, in whichthe material thickness of the pipe is such that a light beam directedacross the pipe and tangent to the internal wall of the pipe, is totallyreflected when the light beam hits the outer surface of the pipe. Thismeans that a light beam which is tangent to the internal wall of thepipe, is refracted in such a way that it is also tangent to the outerwall of the pipe. Given the internal radius r of the pipe, the lightrefraction index n1 of fluid in the pipe and the light refraction indexn2 of the medium externally to the pipe, the external radius R of thepipe may be calculated through the formula R=r*n1/n2 and the materialthickness t=R−r=r(n1/n2−1).

The light refraction index is approximately 1.33 of water and about 1 ofair. For a transparent pipe carrying water and being surrounded by air,R=r*1.33 and the material thickness t=r*0.33. This relationship isindependent of the light refraction index of the pipe material. Thematerial thickness appearing in this way, is greater than what isnormally required for strength considerations in e.g. an apparatus forcounting fish.

The wall of a pipe sized according to the invention, refracts lightbeams which are parallel within the pipe in a plane transverse to thepipe, in towards an optical axis in said plane. The refraction varieswith the distance from the optical axis and in such a way that lightbeams the farthest from the optical axis have the greatest refractionand intersect the optical axis closer to the pipe than light beamscloser to the optical axis. A light beam following the optical axis isnot refracted.

A camera or other optical apparatus for registering, measuring orcounting objects passing in the pipe, may be placed at a camera pointabout midway between the extreme points, wherein light beams that areparallel within the pipe, intersect the optical axis after beingrefracted in connection with the pipe wall. Experiments with a line scancamera have shown surprisingly good results when measuring shadow areason objects illuminated from the opposite side of the pipe. Depending onthe camera and optics it may be necessary to proceed tentatively to thebest camera location in the proximity of said camera point. The lightrefraction index of the pipe material will be of influence.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail in the following by means ofan exemplary embodiment, and reference is made to the appended drawingshowing a cross-section of a transparent pipe with a camera and a lightsource on diametrically opposite sides of the pipe.

DETAILED DESCRIPTION OF THE INVENTION

In the FIGURE the reference numeral 1 identifies a circular pipe of atransparent material, such as glass, acryl or polycarbonate. In the pipe1 water 2 is flowing, carrying objects 3 past a camera 4 which isdirected towards the pipe 1, so that the optical axis 5 of the cameralens is perpendicular to the pipe 1 and passes through the centre of thepipe 1. A light source 6 is placed, relative to the camera 4, on thediametrically opposite side of the pipe 1, so that the camera may beused to measure the shadow area of the objects 3.

The equipment is surrounded by air.

Three pairs of light beams 7, 8 and 9, the light beams of one pair beingon opposite sides of the optical axis 5, show light refraction forparallel light beams within the pipe at a short, mean and maximumdistance, respectively, from the optical axis 5. The course of the lightbeams may be calculated by means of the formula of Snell's law ofrefraction which is well-known to a person skilled in the art. Accordingto the invention, the material thickness of the pipe 1 is selected to besuch that the light beam 9 is tangent to the internal and external wallsof the pipe 1, as explained earlier.

The light beams 7 are refracted and intersect at a point of intersection10, a long distance from the pipe 1. The light beams 8 intersect at apoint of intersection 11, where the camera 4 is located, about midwaybetween the point of intersection 10 and a point of intersection 12,where the light beams 9 intersect one another.

In an experiment, in which the pipe 1 was made of acryl with a lightrefraction index of 1.49, and the pipe had an internal radius of 175millimeters and a material thickness of 58 millimeters, the point ofintersection 10 was about 820 millimeters from the external wall of thepipe 1, whereas the distance to the point 12 was about 440 millimeters.A camera point, the point of intersection 11, at a distance of about 600millimeters from the pipe 1, gave good shadow area measuring results forobjects 3 in an arrangement as shown in the FIGURE.

The point of intersection 10 of the light beams 7 then applied to lightbeams that hit the internal wall of the pipe at an angle of about 5angular degrees relative to the axis of incidence, whereas acorresponding angle of the light beams 9 was then 90 angular degrees.The two angles are shown with reference numerals 13 and 14 in FIG. 1.The camera point refers to the focal point of the camera.

The light source 6 is elongate and arranged to provide diffuse light.The light source 6 comprises a standard fluorescent tube covered by awhite transparent plate which is not shown. The distance between thepipe 1 and the light source 6 may be substantially shorter than thedistance between the pipe 1 and the camera point 11. In the experimentmentioned the shortest distance between the pipe 1 and the light source6 was about 20 millimeters.

The inner and outer walls of the pipe 1 should be designed with smoothsurfaces, so that no scratches or other irregularities will interferewith the light beams. If the camera 4 is a line scan camera, the pipe 1may have a relatively small length, so that in practice the pipe 1 formsa ring.

1. A transparent pipe device of circular cross-section for the opticalcounting and measuring of objects carried with a fluid within the pipe,wherein light beams exiting through the wall of the pipe are sensed byan optical apparatus, and wherein the pipe comprises one in which theouter radius of the pipe equals or nearly equals the product of theinner radius of the pipe and the ratio of the light refraction index ofthe fluid within the pipe to the light refraction index of the mediumsurrounding the pipe.
 2. A device according to claim 1 wherein theoptical apparatus is placed at a camera point at a mean distance fromthe pipe between a distal point of intersection of light beams hittingthe internal wall of the pipe at an angle of about five angular degrees,and a proximal point of intersection of light beams hitting the internalwall of the pipe at an angle of about ninety angular degrees relative tothe axis of incidence.
 3. A device according to claim 1 wherein a lightsource is placed on the diametrically opposite side of the pipe fromsaid optical apparatus and substantially closer to the pipe than theoptical apparatus.